Image processing method and image processing device

ABSTRACT

A threshold value T which is a little larger than voxel values of a target tissue such as bloodstream is determined. Next, a virtual ray is projected, and voxel values on the virtual ray are obtained as an array A 1  (original array). Then, an array A 2  (replaced array) is generated by the voxel values of the array A 1  which are equal to or larger than the threshold value T are flipped-over at the threshold value T. Then, a part of the data on the array A 2 , e.g., flipped-over data corresponding to the center part of the calcified region is excluded. Next, a maximum value M 2  on the array A 2  is obtained, and a value M 1  on the array A 1  corresponding to the value M 2  is obtained. Then, the value M 1  is employed as a pixel value for the virtual ray.

This application is a division of U.S. patent application Ser. No.11/346,058 filed Feb. 2, 2006, which is incorporated herein by referencein its entirety.

This application claims foreign priority based on Japanese Patentapplication No. 2005-054863, filed Feb. 28, 2005, the contents of whichis incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image processing method and imageprocessing device by volume rendering.

2. Description of the Related Art

Hitherto, a projected image has been acquired by projecting virtual rayinto a three-dimensional image obtained with a computed tomography (CT)apparatus, a magnetic resonance imaging (MRI) apparatus, or the like. Asa processing for obtaining such a projected image, volume rendering hasbeen widely employed. As the volume rendering, there are known, forexample, MIP (Maximum Intensity Projection) processing wherein maximumvoxel values are extracted in a projecting direction and are projected,MinIP (Minimum Intensity Projection) processing wherein minimum voxelvalues are extracted and projected, a ray casting method wherein avirtual ray is projected in a projecting direction and a reflected lightfrom an object is calculated, etc.

FIGS. 31A to 31D are explanatory drawings of the MIP processing, andshow a relationship between 3D (three-dimensional) data corresponding tovoxel values of a rendering object and maximum values selected as datafor display. In the MIP processing, since a maximum value of the 3D dataon the projection line shown by an arrow in each figure is used as thedisplay data, 4, 8, 8, and 8, each of which are maximum values of the 3Ddata, are used as the display data in FIGS. 31A, 31B, 31C, and 31D,respectively.

FIG. 32A shows a Raycast image, and FIG. 32B shows an MIP image. TheRaycast image shown in FIG. 32A is one kind of volume rendering image,and pixels are determined by accumulating reflected lights from aplurality of voxels on a virtual ray. Therefore, it is effective inrendering of outlines and a graphical image is obtained. Moreover, inthe case that the virtual ray passes between voxel data, calculation maybe conducted based on not the voxel data themselves but informationobtained by interpolating the voxel data.

On the other hand, the MIP image shown in FIG. 32B is effective inobjectivity and a high speed calculation is possible, since, asmentioned above, the pixels are determined by selecting a single voxelon a virtual ray and the voxel values are rendered as they are.Therefore, the MIP images are frequently used in the rendering of bloodvessels. Sometimes interpolated voxel values are used in MIP processing,and a plurality of voxels are referred to, but there is no difference inthe fact that only the information on a single point on the virtual rayis used. However, sometimes it becomes difficult to render an organhaving no characteristic in the voxel values.

FIGS. 33A and 33B are drawings for illustrating the situation, in an MIPimage, of a portion where a bloodstream 52 is obstructed by a calcifiedregion 50 attached inside a blood vessel. Moreover, FIGS. 33A and 33Bshow the cases that the same portion of the blood vessel is observed inthe directions 90 degrees different from each other.

In the MIP image shown in FIG. 33A, a size of the calcified region 50having a high CT value in the blood vessel can be ascertained. However,the bloodstream 52 in a stenotic portion 51 obstructed with thecalcified region 50 cannot be measured correctly in some cases.Moreover, in the MIP image shown in FIG. 33B, the observation of thebloodstream 52 becomes difficult since the calcified region 50 becomesan obstacle. The bloodstream 52 cannot be observed even when thebloodstream 52 actually positions at the back of or in front of thecalcified region 50.

FIG. 34 is a drawing for explaining characteristics of voxel valuesprofile along a virtual ray, showing a change in voxel values on thevirtual ray at the portion where a calcified region having a high CTvalue exists in a blood vessel. On the virtual ray, the voxel valuescorresponding to the calcified region have large values and showsharp-peaked values. On the other hand, the voxel values of thebloodstream have small values and show smooth-shaped values.

Therefore, in the MIP image, since the maximum value of the voxel valueson the virtual ray is directly displayed, when a blood vessel having acalcified region is observed, the calcified region with a large voxelvalue is displayed, and thus the bloodstream positions at the back of orin front of the calcified region cannot be displayed.

FIG. 35 is a drawing for illustrating a solution of the related art whenthe bloodstream positioning at the back of or in front of the calcifiedregion is observed in the MIP image. As shown in FIG. 35, replacedvolume data is generated by replacing the CT values of the calcifiedregion with some values (e.g., volume data of air). Accordingly, thevoxel values corresponding to the calcified region are lowered so as todisplay the bloodstream. Alternatively, substantially the same effect isobtained by removing a region corresponding to the calcified region fromthe rendering object. However, in the above two methods, it is necessaryto perform a region extraction processing to specify the calcifiedregion in advance.

That is, in the solution of the related art, at the pre-stage of thevolume rendering, a calcified region is detected using a regionextraction method with a threshold value or other algorithms. Then,using the result of the region extraction, the volume data is modified(the calcified region is removed) or mask data is generated(non-rendering region is designated using mask volume) to enable thedisplay of the bloodstream.

FIG. 36 is a flow chart showing calculation of each pixel on the screenin a ray casting method of the related art. In the ray casting method,the following calculation is performed for all the pixels on the screen.First, from the projection position, a projection starting pointO(x,y,z) and a sampling interval ΔS(x,y,z) are set (Step S201).

Next, a reflecting light E is initialized as “0”, a remaining light I as“1”, and a current calculation position X(x,y,z) as “O” (Step S 202).Then, from voxel data in the neighbor of the position X(x,y,z), aninterpolated voxel value V of the position X is obtained (Step S203). Inaddition, an opacity α corresponding to the interpolated voxel value Vis obtained (Step S204). In this case, a function of α=f(V) is preparedbeforehand (Step S 212).

Next, a color value C corresponding to the interpolated voxel value V isobtained (Step S205). Then, from voxel data in the neighbor of theposition X(x,y,z), a gradient G of the position X is obtained, and froma ray direction X−O and the gradient G, a shading coefficient β isobtained (Step S206).

Next, an attenuated light D (D=I*α) and partial reflecting light F(F=β*D*C) at the position X(x,y,z) are calculated (Step S207). Then, thereflecting light E and the remaining light I are updated (I=I−D, E=E+F)(Step S208).

Next, it is determined whether or not X reaches a final position, andwhether or not the remaining light I is “0” (Step S209). When X is notthe final position and the remaining light I is not “0” (no), ΔS(x,y,z)is added to X(x,y,z), the current calculation position is moved on (StepS210), and the processes of and after Step S203 are repeated. On theother hand, when X reaches the final position or the remaining light Iis “0” (yes), calculation is finished with the reflecting light E beingused as the pixel value of the pixel under calculation (Step 211).

FIG. 37 shows a flow chart for calculating each pixel on the screen inan MIP processing of the related art. In the MIP processing, thefollowing calculation is performed for all the pixels on the screen.First, from the projection position, a projection starting pointO(x,y,z) and a sampling interval ΔS(x,y,z) are set (Step S221).

Next, a maximum value M is initialized as a minimum value of the systemand a current calculation position X(x,y,z) as “O” (Step S 222). Then,from voxel data in the neighbor of the position X(x,y,z), a interpolatedvoxel value V of the position X is obtained (Step S223).

Next, the maximum value M and the interpolated voxel value V arecompared (Step S224). When the maximum value M is smaller than theinterpolated voxel value V (yes), the interpolated voxel value V isassigned to the maximum value M as a new Maximum value (Step S225).Then, it is determined whether or not the current calculation position Xreaches a final position (Step 226). When the current calculationposition X is at the final position (no), ΔS(x,y,z) is added toX(x,y,z), the current calculation position is moved on (Step S227), andthe processes of and after Step S223 are repeated. On the other hand,when the current calculation position X reaches the final position(yes), the maximum value M is used as the pixel value of the pixel undercalculation (Step 228).

Moreover, in U.S. Pat. No. 6,205,350, second volume data not containingan obstructing region is generated, the maximum value in the secondvolume data is obtained, and a value in the original volume data at theposition corresponding to the position of the maximum value is used forrendering.

However, in the above methods of the related art, obstructing regionssuch as calcified regions are removed by the replacement of volume data.Hence the information of the obstructing regions themselves iscompletely lost. Moreover, it is difficult to exclude exactly only theobstructing region and to render the bloodstream correctly. Furthermore,since an extracted region is designated in voxel units, aliases mayarise at the boundary of the region, which results in deterioration ofthe image. In addition, retention of mask information and second volumedata may cause an unnecessary load to the memory, and when the volumedata is modified, the comparison with the original date becomesdifficult. Additionally, the extraction of individual obstructingregions takes much time and largely depends on subjective factors of auser. In particular, since the extraction depends on the subjectivefactors of the user, reproducibility by each user is low, which resultsin lack of universality as objective diagnostic information. Therefore,there is a problem that it is difficult to use the methods at actualdiagnosis and hence actually, they are not so widely employed.

FIGS. 38A, 38B and 38C are drawings for illustrating the problems in theMIP image of the related art. In the method of the related art, as shownin FIGS. 38A and 38B, a calcified region 61 is removed in order toobserve a bloodstream 60 at the back of and in front of the calcifiedregion 61. In that case, a portion 62 where the bloodstream 60 exists isalso removed. Moreover, in the method of the related art, the calcifiedregion 61 is not displayed at all, and hence it becomes difficult todetermine a diseased part. Also a necessary region is removedfrequently, and hence reliability decreases.

In this case, as shown in FIG. 38C, information of a portion 63 which isan imprint of the removed calcified region is necessary. Particularly,information of an outline portion 64 of the calcified region isrequired. That is, when only the outline of the calcified region isdisplayed without displaying the filling of the calcified region, thedisplay is effective for diagnosis.

In this regard, since the calcified region is a three-dimensionalregion, the boundary surface of the region constitutes a curved surfacein a three-dimensional space. Therefore, when the calcification isrendered by a mask application or a volume modification of the relatedart, each pixel of an image represents an intersection of a virtual rayand the three-dimensional boundary surface, the intersectionconstituting the pixel, so that a two-dimensional outline cannot berepresented. On the other hand, when diagnosis is conducted whileviewing the image, for the calculation of images, information of thetwo-dimensional outline portion of the calcified region and the neighborof the calcified region, particularly at the back of and in front of thecalcified region is necessary. With regard to the two-dimensionaloutline portion, when only the portion can be rendered where the virtualray grazes the rim of the calcified region three-dimensionally, therendering is effective for diagnosis.

SUMMARY OF THE INVENTION

An object of the invention is to provide an image processing method andimage processing device in which a rendering is possible by determininga two-dimensional outline of an obstructing region dynamically, whileremoving the obstructing region such as a calcified region, whencalculation such as MIP processing is performed for a medical image.

An image processing method of the invention is an image processingmethod by volume rendering, comprising: selecting at least one pointwhich is aligned on a virtual ray; and determining a pixel value of animage by the volume rendering based on each value of the selected atleast one point, wherein said at least one point is selected based on afirst vector information and a second vector information, and positionalrelationship of said at least one point is mutually exchangeable on thevirtual ray in determining the pixel value. In the related art, in thevolume rendering method without calculating a reflecting light, such asMIP method, vector information is not used. However, according to theabove configuration, by using the vector information, for example, atwo-dimensional effect (two-dimensional outline) depending on thedirection of the virtual ray can be added to the image. Thereby, itbecomes possible to observe, for example, shape information of anobstructing region and information of the neighbor thereof at one time,utilizing the two-dimensional outline.

The processing object of the image processing method of the invention isvolume rendering. In the above expression “positional relationship of atleast one point is mutually exchangeable on the virtual ray”, “point” isused instead of “voxel” because, in many cases, pixel values arecalculated based not on voxel values but on values obtained byinterpolating the voxel values. Furthermore, in the expression, “atleast one point” is used instead of “one point” because the inventioncan be applied not only to MIP method where only one point on a virtualray is used, but also to other methods such as Top10MIP method, in whichan average value of top ten points on a virtual ray is displayed. TheMIP method is included in an image processing method of the presentinvention, since even when volume data is inversed in a direction ofdepth, the same image is obtained. In the MIP method and the like, sincethe positional relationship of points on the virtual ray is not used, avalue such as opacity cannot be defined. This situation is the same inan average value method which uses an average value of points on avirtual ray, since even when the positions of the points are mutuallyexchanged, the result is not affected. Also in a method using a weightedaverage of points on a virtual ray, even when the positions of thepoints are mutually exchanged along with their associated weights (ordegree of contribution of each point on a pixel), the result is notaffected. Accordingly, in an image processing method in the presentinvention, the degree of contribution may be multi-valued. In a volumerendering method to which the present invention is applicable, gradientof a voxel is hitherto not considered in computation, since such methodis not based on a simulation of a light ray. To the contrary, in raycasting method, opacity is associated with each voxel and attenuation oflight amount of a virtual ray passing through voxels is processed.Accordingly, a different image is obtained when back and front of thevolume data are inverted.

Moreover, the first vector information is mainly a direction vector ofthe virtual ray, but when other direction vector is used, an imagecorresponding to the other direction vector is obtained. On the otherhand, the second vector information is mainly a gradient of voxel(including interpolated gradient), but other vector information relatedto voxels, such as movement information, can be used.

Moreover, in the image processing method of the invention, the firstvector information is a direction vector of the virtual ray. In theimage processing method of the invention, the second vector informationis gradient information. In the image processing method of theinvention, a number of the selected point is one.

In the image processing method of the invention, said at least one pointis selected further based on data obtained by replacing original data onthe virtual ray. In the image processing method of the invention, valuesof the replaced data are obtained by flipping values of the originaldata over at a threshold value.

In the image processing method of the invention, said at least one pointis selected further based on a magnitude of the second vectorinformation. In the image processing method of the invention, said atleast one point is selected further based on an angle between the firstvector information and the second vector information. The imageprocessing method of the invention further comprises displaying atwo-dimensional outline of a region included in a rendering object onthe volume rendering image.

Further an image processing method of the invention is an imageprocessing method by volume rendering, comprising: selecting at leastone point which is aligned on a virtual ray; determining a degree ofcontribution of each value of the selected at least one point; anddetermining a pixel value of an image by the volume rendering based onthe determined degree of contribution and said each value of theselected at least one point.

In the volume rendering method of the related art, the degree ofcontribution on the processing for determining pixel values is providedthrough mask information or the like. Thus, the degree of contributionis three-dimensionally determined and a two-dimensional effect cannot beadded. To the contrary, according to the above configuration, since thedegree of contribution is determined on the virtual ray, thetwo-dimensional effect can be added to the image.

Moreover, in the image processing method of the invention, at least oneof the degree of contribution is zero. In the image processing method ofthe invention, the degree of contribution is determined based on dataobtained by replacing original data on the virtual ray. In the imageprocessing method of the invention, values of the replaced data areobtained by flipping values of the original data over at a thresholdvalue.

Furthermore, in the image processing method of the invention, the degreeof contribution is determined further based on a gradient vector whichis on a volume and corresponds to a position of the selected point, anda direction vector of the virtual ray. In the image processing method ofthe invention, the degree of contribution is determined further based ona change of voxel values on the virtual ray.

In addition, the image processing method of the invention furthercomprises displaying a two-dimensional outline of a region included in arendering object on the volume rendering image, based on the determinedpixel value. The image processing method of the invention furthercomprises displaying excluding a region included in a rendering objecton the volume rendering image.

Moreover, in the image processing method of the invention, the volumerendering image and an another image are displayed arranged in side byside, being overlapped with each other, or by showing a difference ofthe images. In the image processing method of the invention, the pixelvalue is determined only for a region which is designated by a user. Inthe image processing method of the invention, the pixel value isdetermined only for a window provided on a screen.

Furthermore, in the image processing method of the invention, theoutline is displayed while continuously changed. In the image processingmethod of the invention, the image processing is performed by parallelprocessing. In the image processing method of the invention, the imageprocessing is performed by a GPU (graphics processing unit). In theimage processing method of the invention, the image processing isperformed by a GUI (graphical user interface) in which parameters arechangeable.

In addition, in the image processing method of the invention, the imageprocessing is performed by MIP (Maximum Intensity Projection) method,MinIP (Minimum Intensity Projection) method, Raysum method or an averagevalue method. In the image processing method of the invention, the imageprocessing is performed by MIP (Maximum Intensity Projection) method,MinIP (Minimum Intensity Projection) method, Raysum method, an averagevalue method or ray casting method. The image processing method of thepresent invention further comprises displaying the selected at least onepoint on a sectional image of a rendering object, said sectional imageincluding the virtual ray.

Furthermore, an image processing device in the invention is an imageprocessing device for displaying a volume rendering image, beingoperative to: select at least one point which is aligned on a virtualray; determine a pixel value of an image by the volume rendering basedon each value of the selected at least one point; and display atwo-dimensional outline of a region included in a rendering object onthe volume rendering image, wherein positional relationship of said atleast one point is mutually exchangeable on the virtual ray indetermining the pixel value.

According to the image processing method of the invention, at the timewhen a virtual ray is projected, a diagnostic object can be accuratelydisplayed by determining parts of voxel data in the voxel data on thevirtual ray, e.g., a two-dimensional center part of an obstructingregion, as non-display data, and determining a two-dimensional outlineof the obstructing region as display data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are drawings for explaining characteristics of voxelvalues profile along a virtual ray used in the image processing methodaccording to embodiments of the invention.

FIG. 2 is a drawing for explaining characteristics of voxel valuesprofile along a virtual ray, showing changes of voxel values when thevirtual ray passes through the center part of a calcified region 20, rimof the calcified region 21, and a bloodstream 22.

FIG. 3 is a drawing for explaining characteristics of voxel valuesprofile along a virtual ray, showing the case when gradient is used inthe MIP processing (without flip-over) in the image processing accordingto a first embodiment of the embodiment.

FIGS. 4A and 4B are explanatory drawings of a determining method (1)whether voxel values corresponding to the virtual ray are excluded inthe image processing method according to a first embodiment of theinvention.

FIGS. 5A and 5B are explanatory drawings of the case when whether voxelvalues corresponding to the virtual ray are excluded from the displaydata is determined for the region in the group to be possibly excludedin an image processing method according to a first embodiment of theinvention.

FIG. 6 is a flow chart of MIP processing (without flip-over) usinggradient in the image processing method according to an embodiment ofthe invention.

FIGS. 7A, 7B and 7C are drawings for explaining characteristics of voxelvalues profile along a virtual ray, showing the case when gradient andflip-over are used for the MIP processing in the image processing methodaccording to a second embodiment of the invention.

FIGS. 8A, 8B and 8C are drawings for explaining characteristics of voxelvalues profile along a virtual ray, showing modification of a profilepattern with a gradient factor.

FIGS. 9A and 9B are drawings for explaining characteristics of voxelvalues profile along a virtual ray, showing modification of a profilepattern with flip-over.

FIG. 10 is an explanatory drawing of examples of LUT function and adisplay of a calcified region.

FIG. 11 shows a flow chart of MIP processing using gradient andflip-over in the image processing method according to a secondembodiment of the invention.

FIG. 12 is a drawing for explaining characteristics of voxel valuesprofile along a virtual ray, showing a procedure 1 wherein the centerpart of a calcified region is removed, and the bloodstream in front ofand at the back of the center part of the calcified region is displayedas well as the rim of the calcified region, in the image processingmethod according to a third embodiment of the invention.

FIGS. 13A to 13C are drawings for explaining characteristics of voxelvalues profile along a virtual ray, showing profile patterns (1)obtained by the image processing method according to a third embodimentof the invention.

FIG. 13D shows an image displayed by the image processing methodaccording to a third embodiment of the invention.

FIGS. 14A to 14C are drawings for explaining characteristics of voxelvalues profile along a virtual ray, showing profile patterns (2)obtained by the image processing method of according to a thirdembodiment of the invention.

FIG. 14D shows an image displayed by the image processing methodaccording to a third embodiment of the invention.

FIG. 15 is a drawing for explaining characteristics of voxel valuesprofile along a virtual ray, showing a procedure 2 in the imageprocessing method according to a third embodiment of the invention.

FIGS. 16A and 16B are drawings for explaining characteristics of voxelvalues profile along a virtual ray, showing a determining method (2) inprocedure 2 for determining whether a voxel value corresponding to thevirtual ray is to be excluded or not, in the image processing methodaccording to a third embodiment of the invention.

FIG. 17 is a drawing for explaining characteristics of voxel valuesprofile along a virtual ray, showing a determining method (3) inprocedure 2 for determining whether a voxel value corresponding to thevirtual ray is excluded or not, in the image processing method accordingto a third embodiment of the invention.

FIGS. 18A and 18B are drawings for explaining characteristics of voxelvalues profile along a virtual ray, showing the case when the rim of thecalcified region is detected employing a transformation method otherthan the flip-over which uses a threshold value in the image processingmethod according to a fourth embodiment of the invention.

FIGS. 19A and 19B are drawings for explaining characteristics of voxelvalues profile along a virtual ray, showing a procedure 3 in the casewhen the original data before flip-over is employed as display data ofthe rim of the calcified region in the image processing method accordingto a fifth embodiment of the invention.

FIGS. 20A and 20B are drawings for explaining characteristics of voxelvalues profile along a virtual ray, showing preferable examples in theimage processing method according to a sixth embodiment of theinvention.

FIG. 21A shows an example of an MIP image of the related art.

FIG. 21B shows an example of an MIP image generated by the imageprocessing method according to embodiments of the invention.

FIG. 22 shows a flow chart illustrating q processing for obtaining eachpixel value of an image in the image processing method according to asixth embodiment of the invention.

FIG. 23 shows a flow chart of the case when whether data is to beexcluded is determined using a change in voxel data in the imageprocessing method according to a sixth embodiment of the invention.

FIG. 24 shows a flow chart of the case when exclusion of data isdetermined using gradient in the image processing method according to asixth embodiment of the invention.

FIG. 25 is an explanatory drawing of the case when the opacity ischanged (Application 1 for ray casting method) in the image processingmethod according to an seventh embodiment of the invention.

FIG. 26 shows a flow chart of the case when the opacity is changed usinggradient (Application 1 for ray casting method) in the image processingmethod according to an seventh embodiment of the invention.

FIG. 27 is a drawing for explaining characteristics of voxel valuesprofile along a virtual ray, showing the case when the opacity ischanged (Application 2 for ray casting method) in the image processingmethod according to a eighth embodiment of the invention.

FIG. 28 shows a flow chart of the case when the opacity is changed usingthe change in voxel values (Application 2 for ray casting method) in theimage processing method according to a eighth embodiment of theinvention.

FIG. 29 shows a flow chart of MIP processing in which gradientprocessing and flip-over processing are conducted in the imageprocessing method according to a ninth embodiment of the invention.

FIGS. 30A and 30B are explanatory drawings of the case when the crossingof bloodstreams are visualized.

FIGS. 31A, 31B, 31C and 31D are explanatory drawings of MIP processingon voxel values and 3D data.

FIG. 32A is an example of a Raycast image.

FIG. 32B is an example of an MIP image.

FIGS. 33A and 33B are drawings for illustrating the situation, in an MIPimage, of a portion where a bloodstream 52 is obstructed by a calcifiedregion 50 attached inside a blood vessel.

FIG. 34 is a drawing for explaining characteristics of voxel valuesprofile along a virtual ray, showing a change in voxel values on thevirtual ray at the portion where a calcified region having a high CTvalue exists in a blood vessel.

FIG. 35 is a drawing for explaining characteristics of voxel valuesprofile along a virtual ray, illustrating a solution of the related artwhen the bloodstream positioning at the back of or in front of thecalcified region is observed in the MIP image.

FIG. 36 is a flow chart showing calculation of each pixel on the screenin a ray casting method of the related art.

FIG. 37 shows a flow chart for calculating each pixel on the screen inan MIP processing of the related art.

FIGS. 38A, 38B and 38C are drawings for illustrating the problems in theMIP image of the related art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1A and 1B show a change (profile pattern) of voxel values(including interpolated voxel values) on a virtual ray when the virtualray passes through volume data in the image processing method of thepresent embodiments. The profile pattern is determined with everyvirtual ray, and has a characteristic for each object through which thevirtual ray passes. Here, a profile pattern is shown of the voxel valueswhen the virtual ray passes through an obstructing region such as acalcified region. When the virtual ray passes through the center part ofthe obstructing region 10 as shown in FIG. 1A, the voxel valuescorresponding to the obstructing region 10 jump remarkably. On the otherhand, when the virtual ray grazes the rim 11 of the obstructing region10 as shown in FIG. 1B, increase of the voxel values corresponding tothe obstructing region 10 is limiting and relatively flat.

FIG. 2 shows a profile pattern of voxel values when a virtual ray passesthrough the center part and a rim 21 of a calcified region 20, and abloodstream 22. When the virtual ray passes through the center part ofthe calcified region 20, the voxel values have a high peak (maximumvalue). When the virtual ray passes through the rim 21 of the calcifiedregion 20, the voxel values have a low peak. Moreover, the voxel valuescorresponding to the bloodstream 22 have a flat hill-like pattern.

First Embodiment

(MIP Processing—Gradient is Used without Flip-Over)

FIG. 3 is an explanatory drawing of the case when gradient is used inthe MIP processing (without flip-over) in the image processing of thepresent embodiment. In the present embodiment, two threshold values areprepared, and voxel values are separated into three groups by thethreshold values in order to perform calculation for exclusion of anobstructing region. That is, the voxel values are separated into threegroups: a group to be completely excluded (a portion having sufficientlyhigh voxel values is regarded as a calcified region and removed); agroup to be possibly excluded (a region where voxel values do not givesufficient clues for determining whether the region should be removed ornot); and a group not to be excluded (normal tissue). Then only theregion in the group to be possibly excluded is separated again based onthe gradient.

In the present embodiment, both of the calcified region itself and thetwo-dimensional outline of the calcified region can be determined byusing the direction vector of the virtual ray and the gradientinformation of voxels through which the virtual ray passes. The gradientinformation of the voxels can be obtained by acquiring the difference ofneighboring 3×3×3 voxel region of a target voxel.

On the other hand, in the related art, voxel values having equal to orlarger than a certain threshold value are automatically removed, andhence the maximum value on the virtual ray is necessarily fixed to thethreshold value. In that case, the bloodstream or the like existing atthe back of or in front of the calcified region can not be observed. Inthe present embodiment, an intermediate range of voxel value isprovided, and for a voxel having value in the intermediate range,whether the voxel belongs to the calcified region or not is determinedby using the gradient, whereby a maximum value of a desired portion isobtained.

FIGS. 4A and 4B are explanatory drawings of a determining method (1)whether voxel values corresponding to the virtual ray are excluded inthe image processing method of the present embodiment. In this case, agradient vector G of three-dimensional voxel values is used as adetermination method for excluding the center part of a calcified regionand employing only the rim of the calcified region.

As shown in FIG. 4A, when the virtual ray passes through the center partof the calcified region 20, the gradient vector G 30 of the voxelbecomes substantially parallel to the advancing direction of the virtualray. Therefore, the portion where the gradient vector G 30 issubstantially parallel to the advancing direction of the virtual ray isdetermined to be the center part of the calcified region 20, and thusthe portion is excluded from display data.

On the other hand, as shown in FIG. 4B, when the virtual ray grazes therim 21 of the calcified region, the gradient vector G 31 of the voxelbecomes substantially perpendicular to the advancing direction of thevirtual ray. Therefore, the portion where the gradient vector G 31 issubstantially perpendicular to the advancing direction of the virtualray is determined to be the rim 21 of the calcified region, and thus theportion is employed as display data.

In this case, determination for excluding or employing the display datacan be conducted as follows: an inner product |D·G| is obtained by avirtual ray vector D and the gradient vector G in a candidate positionof the maximum value of the voxel values; then, the absolute value |D·G|is compared with a threshold value of permissible parallelism TP (to beprecisely described in FIG. 24). Moreover, the threshold value ofpermissible parallelism TP can be dynamically changed through GUI(Graphical User Interface) by a user, and the degree of removal of thecalcified region can be changed while viewing the displayed image.

FIGS. 5A and 5B are explanatory drawings of the case when whether voxelvalues corresponding to the virtual ray are excluded from the displaydata is determined for the region in the group to be possibly excluded,in an image processing method of the present embodiment. FIG. 5A showsthe case when the virtual ray passes through the center part of theobstructing region. FIG. 5B shows the case when the virtual ray passesthrough the outline of the obstructing region. The determination whetherthe exclusion is possible for the region in the group to be possiblyexcluded is performed by using an evaluation function f using thegradient vector G and a virtual ray vector D. That is, when thefollowing conditional expression:

f(G,D)>a threshold value

is satisfied, it is determined that the exclusion is possible. Forexample, the expression may be:

f(G,D)=(1−(1−G·D/(|G|·|D|))̂2*h(|G|)) (G·D is Inner Product of Vector Gand D)

wherein 1−(1−G·D/(|G|·|D|)) is a value relating to an intersecting angleof the gradient vector G and the virtual ray vector D, and h(|G|) is anormalized value of the gradient vector G.

FIG. 6 is a flow chart of MIP processing (without flip-over) usinggradient in the image processing method of the present embodiment. Thisis a method for calculating each pixel on the screen, and the followingcalculation is conducted for all the pixels on the image. First, from aprojection position, a projection starting point O(x,y,z) and a samplinginterval ΔS(x,y,z), a virtual ray direction D(x,y,z) are set (StepS301).

Next, a maximum value M is initialized as a minimum value of the systemand a current calculation position X(x,y,z) as “O” (Step S302). Then,from voxel data in the neighbor of a position X(x,y,z), an interpolatedvoxel value V of the position X is obtained (Step S303).

Next, the value of V is checked (Step S304), and when the value of V isdetermined as “possibly excluded”, the gradient G at the position X iscalculated (Step S305). Then, the gradient G is compared with athreshold value T (Step S306), and when f(G,D) is equal to or smallerthan the threshold value T (no), M and V are compared (Step S307). WhenM<V is true (yes), V is assigned to M as a new Maximum value (StepS308).

Next, it is determined whether X reaches a final position or not (StepS309). When X is not at the final position (no), ΔS(x,y,z) is added toX(x,y,z), the current calculation position is moved on (Step S310), andthe processes of and after Step S303 are repeated. On the other hand,when X reaches the final position (yes), the maximum value M is used asa pixel value of the pixel under calculation (Step 311).

Second Embodiment

(MIP Processing—Gradient is Used with Flip-Over)

FIGS. 7A, 7B and 7C are explanatory drawings of the case when gradientand flip-over are used for the MIP processing in the image processingmethod of the present embodiment. In the image processing method of thepresent embodiment, a first profile pattern is modified using“flip-over” and “gradient factor”. Thereby, the position of the maximumvalue changes, and hence the changed maximum value is utilized. That is,a second profile pattern can be generated by replacing the data on thefirst profile pattern. By conducting such processing, as compared withthe case when the exclusion is performed just by determining a rangewith thresholds for removing an obstacle on the virtual ray, smootherchange can be applied to the image. Thus, in rendering, it can beprevented that the boundary of the excluded region appears as alias.Furthermore, more flexibility can be enhanced in the image processing.

In the present embodiment, the first profile pattern is processed togenerate the second profile pattern, and a maximum value is obtained onthe second profile pattern. In the MIP processing of the related art,the maximum value is obtained on the first profile pattern. However,since the maximum value does not necessarily correspond to the portionto be observed, sometimes the calcified region is displayed on thescreen as a result. On the other hand, when the replacement isconducted, as the position of the maximum value moves on the profilepattern, a more suitable value may be employed as the maximum value.

FIGS. 8A, 8B and 8C are explanatory drawings showing modification of aprofile pattern with a gradient factor. Moreover, FIGS. 9A and 9B areexplanatory drawings showing modification of a profile pattern withflip-over. The transformation of the profile pattern equals totransforming each point on the profile pattern by a function. That is,assuming that the voxel value is V, a flipped-over voxel value is flipV,the virtual ray vector is R, the gradient G=grad(V), the magnitude ofthe gradient L=|G|, and the angle with the virtual rayθ=arccos(G·R/(|G|·|R|)), a voxel value V2 of the transformed profilepattern is calculated as V2=f(flipV, L, θ). In this case, f may be anyfunction. Here, f(flipV, L, θ)=flipV*LUT(L, θ): lookup table isexemplified.

FIG. 10 is an explanatory drawing of examples of LUT function and adisplay of a calcified region. In the usual display of a calcifiedregion, the outline is unclear. However, in the display of a calcifiedregion by the present method, the outline is clear. Also, more desirableimage is obtained by considering both the direction and magnitude of thegradient as shown in FIG. 10. Furthermore, the LUT function isexemplified with a binary function for explanation, but the LUT functionmay be a multi-valued function. Thereby, a smoother result image isobtained.

FIG. 11 shows a flow chart of MIP processing using gradient andflip-over in the image processing method of the present embodiment. Thisis a method for calculating each pixel on the screen, and the followingcalculation is conducted for all the pixels on the image. First, from aprojection position, a projection starting point O(x,y,z), a samplinginterval ΔS(x,y,z) and a virtual ray direction D(x,y,z) are set (StepS321).

Next, a maximum value M is initialized as a minimum value of the system,a maximum value M2 as the minimum value of the system, a position of themaximum value VX is initialized, a current calculation position X(x,y,z)is initialized as “O”, and a flip-over threshold T is set (Step S322).Then, from voxel data in the neighbor of the position X(x,y,z), aninterpolated voxel value V of the position X is obtained (Step S323).

Next, the gradient G of the position X is calculated (Step S324),flipV=flip-over function(V,T) is calculated (Step S325), and L=|G| iscalculated (Step S326). Then, θ=arccos(G·R/(|G|·|R|)) is calculated(Step S327), and V2=f(flipV, L, θ) is calculated (Step S328). Here, theprofile pattern may be dynamically calculated, not being preparedbeforehand.

Next, M2 and V2 are compared (Step S329) and when M2<V2 is true (yes), Vis assigned to M as a new Maximum value and V2 is assigned to M2 as anew Maximum value (Step S330). Then, whether X reaches a final positionor not is determined (Step S331). When X is not at the final position(no), ΔS(x,y,z) is added to X(x,y,z), the current calculation positionis moved on (Step S332), and the processes of and after Step S323 arerepeated. On the other hand, when X reaches the final position (yes),the maximum value M is used as a pixel value of the pixel undercalculation (Step 333).

Third Embodiment

(MIP Processing—with Flip-Over)

In the image processing method of the present embodiment, a processingis performed in which a magnitude of a local slope of the profilepattern is calculated for determining whether the calcified region is tobe excluded. Furthermore, in order to elicit the magnitude of the localslope, a second profile pattern is generated. FIG. 12 shows a procedure1 of the present embodiment, wherein the second profile pattern isobtained by replacing the original profile pattern of the virtual rayhaving equal to or larger than a threshold value, with flip-over. Inprocedure 1, for example, the second profile pattern is generated, whichis a replacing data, by flipping over the profile pattern having voxelvalues equal to or larger than a threshold value (threshold value T: tobe precisely described in FIG. 22). This procedure prevent rendering ofa calcified region having high voxel values, and is conducted forexcluding the portion where change in the voxel values is large in thenext procedure 2, and further, is for highlighting the outline of arendering object. As a characteristic of the present processing, it isnot required to obtain the boundary of a calcified region strictly.Moreover, being compared with the method using a gradient, since thepresent processing is completed only with the data on the virtual ray,calculation is simple and is conducted at a high speed.

Thereby, in the image processing method of the present embodiment, theoutline can be rendered, and rendering of calcified region having highvoxel values can be prevented. Moreover, the maximum value (thresholdvalue T) can be easily set, and rendering can be performed with a highreproducibility. Furthermore, the threshold value T can be dynamicallychanged though GUI by a user, and the user can change the degree ofremoval of the calcified region while viewing the displayed image.

FIGS. 13A to 13C show profile patterns (1) obtained by the imageprocessing method of the present embodiment, and FIG. 13D shows an imagedisplayed by the image processing method. The profile pattern representsvoxel values (inclusive of interpolated voxel values) on the virtualray. That is, as shown in FIG. 13A, on the virtual ray passing through abloodstream 22, voxel values corresponding to the bloodstream 22 appearas the profile pattern. Moreover, as shown in FIG. 13B, on the virtualray passing through a rim 21 of a calcified region, voxel valuescorresponding to the rim 21 of the calcified region appear as theprofile pattern. Furthermore, as shown in FIG. 13C, on the virtual raypassing through the center part of the calcified region 20, voxel valuescorresponding to the bloodstream 22 instead of the calcified region 20appear as the profile pattern.

Therefore, in the displayed image, as shown in FIG. 13D, the bloodstream22 and the rim (outline) 21 of the calcified region are displayedwithout displaying the center part of the calcified region 20. Also, inthe region of the center part of the calcified region 20, thebloodstream 22 exists at the back of the calcified region 20 can bedisplayed. Accordingly, the size (outline) of the calcified region 20and the bloodstream 22 of blood vessel narrowed by the calcified region20 can be obtained by the user at the same time.

FIGS. 14A to 14C show profile patterns (2) obtained by the imageprocessing method of the present embodiment, and FIG. 14D shows an imagedisplayed by the image processing method. That is, as shown in FIG. 14A,when the bloodstream 22 hardly exists, voxel values as same as those ofneighboring tissue are employed. Moreover, as shown in FIG. 14B, for thevirtual ray passing through the rim 21 of a calcified region, voxelvalues corresponding to the rim 21 of the calcified region are employed.Furthermore, as shown in FIG. 14C, for the virtual ray passing throughthe center part of the calcified region 20, a maximum value other thanthe excluded range, for example, voxel value corresponding to theneighborhood of the calcified region 20 is employed, since thebloodstream 22 does not exists in front of or at the back of thecalcified region 20. Thereby, a user can precisely obtain the size(outline) of the calcified region 20 while viewing the image.

FIG. 15 shows a procedure 2 in the image processing method of thepresent embodiment. In the procedure 2, in order to exclude the portionhaving a large change from the rendering object, for example, a portionon the virtual ray where the absolute value of second order derivativeis larger than the threshold value is excluded. Thereby, the rim of thecalcified region is rendered so as to obtain the size of the calcifiedregion. In addition, the bloodstream in front of and at the back of thecalcified region can be rendered, which is difficult to be rendered inthe related art. This processing avoids that, after the flip-overprocessing in procedure 1, the threshold value at which voxel values areflipped-over is necessarily selected as a maximum value on the virtualray.

FIGS. 16A and 16B are explanatory drawings of determining method (2) inprocedure 2 for determining whether a voxel value corresponding to thevirtual ray is to be excluded. FIG. 16A shows a profile of voxel valuesin the case when the virtual ray passes through the rim of the calcifiedregion. In this case, for example, the magnitude of vertical change (inthe direction of the voxel value axis) of the voxel values, or slope ofthe voxel value profile is measured. Then, for example, the measuredmagnitude of vertical change in the voxel values, or the measured slopeof the voxel value profile is compared with a permissible changethreshold value TD (to be precisely described in FIG. 23). Accordingly,it is detected that the region through which the virtual ray passes isthe rim of the calcified region, and the voxel is determined as data tobe displayed.

On the other hand, FIG. 16B shows a profile pattern in the case when thevirtual ray passes through the center part of the calcified region. Inthis case, for example, the magnitude of vertical change in the voxelvalues, or the magnitude of gradient of the voxel values is measured.Accordingly, it is detected that the region through which the virtualray passes is the center part of the calcified region. Then, the voxelsin a certain range on the profile pattern are excluded from the data tobe displayed. Thereby, only the rim, i.e., the outline of the calcifiedregion can be rendered without rendering the center part of thecalcified region. Furthermore, the bloodstream existing in front of andat the back of the center part of the calcified region can be rendered.Moreover, the permissible change threshold value TD can be dynamicallychanged though GUI by a user, and the user can change the degree ofremoval of the calcified region while viewing the displayed image.

FIG. 17 is an explanatory drawing of determining method (3) in procedure2 for determining whether a voxel value corresponding to the virtual rayis excluded, and shows a profile of voxel values in the case when thevirtual ray passes through the rim of the calcified region. In thiscase, the magnitude of the gradients before and after a flip-over point(spike) is referred to. Since only one of the gradients before and afterthe flip-over point is large, this portion is determined as the rim ofthe calcified region, and is employed as display data. Forsimplification, second order derivative values may be obtained using thevoxel values before and after the flip-over point instead of referring 2gradients. Conditional expression using Second order derivative may begiven by the following.

p0−2*p1+p2>threshold

Fourth Embodiment

(MIP Processing—with Transformation with Threshold Cutting)

In the second and the third embodiments, flip-over processing isperformed on a voxel value profile. Alternatively, other transformationmethod may be applied to the profile. FIGS. 18A and 18B show explanatorydrawings of the case when the rim of the calcified region is detectedemploying a transformation method other than the flip-over. In thisembodiment, the voxel values are not flipped over at a threshold value,but the voxel values are rounded at a threshold value. Then, gradient ofthe rounded voxel value is calculated, and the voxel is determined to beexcluded from or employed in display data by the change in the gradient.

That is, as shown in FIG. 18A, when the gradient of the voxel valuesrounded at a certain threshold is small, the rounded region isdetermined as the rim of the calcified region, and is employed asdisplay data. On the other hand, as shown in FIG. 18B, when the gradientof the voxel values rounded at a certain threshold is large, the roundedregion is determined as the center part of the calcified region, and isexcluded from the display data. According to this method, it is possibleto determine whether the voxel is to be excluded from or employed in thedisplay data without the flip-over process at a threshold value.

Fifth Embodiment

(MIP Processing—Original Data is Used with Flip-Over)

In the second and the third embodiments, flip-over processing isperformed on a voxel value profile, and data on the transformed profileis selected as a display data. However, it is preferable that thedisplay data is obtained from the original profile rather than thetransformed profile. FIGS. 19A and 19B show a procedure 3 in the casewhen the original data before flip-over is employed as display data ofthe rim of the calcified region in the image processing method of thepresent embodiment. In the present embodiment, the user can dynamicallyset the threshold of the flip-over processing through GUI to make itclose to the maximum value in the region to be displayed. However,depending on the position through which the virtual ray passes, thevoxel values corresponding to the rim of the calcified region may becomean object to be flipped over as shown in FIG. 19B. In this case, asshown in FIG. 19A, by using the voxel values (original data)corresponding to the rim of the calcified region before the flip-overprocessing, the rim of the calcified region can be clearly rendered.

Sixth Embodiment

(Preferred Embodiment—MIP Processing with Flip-Over)

FIGS. 20A and 20B show preferable examples in the image processingmethod of the present embodiment. FIG. 20A shows a case when the centerpart of the calcified region, the rim of the calcified region, and thebloodstream exist on the virtual ray. By performing the flip-overprocessing at a threshold value to detect the magnitude of change invoxel values, it is determined whether the region through which thevirtual ray passes is the center part of the calcified region or the rimof the calcified region. Therefore, only the rim of the calcified regioncan be displayed without displaying the center part of the calcifiedregion.

Moreover, FIG. 20B is a case when the center part of the calcifiedregion and the bloodstream exist on the virtual ray. By performing theflip-over processing at a maximum value to detect the magnitude ofchange in voxel values, the center part of the calcified region isdetermined. Therefore, only the bloodstream existing at the back of thecenter part of the calcified region can be displayed without displayingthe center part.

FIG. 21A shows an example of an MIP image of the related art, and FIG.21B shows an example of an MIP image generated by the image processingmethod of the present embodiment. In the MIP image shown in FIG. 21A,the bloodstream in front of and at the back of the calcified region isnot clear by the presence of the calcified region. However, the MIPimage of the present embodiment shown in FIG. 21B, only the outline ofthe calcified region is displayed, and the user can obtain whether thebloodstream remains or not in front of and at the back of the calcifiedregion.

FIG. 22 shows a flow chart illustrating an entire picture of theprocessing for obtaining each pixel value of an image in the imageprocessing method of the present embodiment. In order to obtain eachpixel value of an image, at first, as shown in FIG. 12, the thresholdvalue T which is a little larger than the voxel values of a targettissue such as bloodstream is determined (Step S11).

Next, a virtual ray is projected (Step S12), and voxel values on thevirtual ray are obtained as an array A1 (original (first) profilepattern) (Step S13). Then, an array A2 (second profile pattern) isgenerated by the voxel values of the array A1 which are equal to orlarger than the threshold value T are flipped-over at the thresholdvalue T (Step S14). Then, a part of the data on the array A2, e.g.,flipped-over data corresponding to the center part of the calcifiedregion is excluded (Step S15).

Next, a maximum value M2 on the array A2 is obtained (Step S16), and avalue M1 on the array A1 (original data in FIG. 19A) corresponding tothe value M2 is obtained (Step S17). Then, the value M1 is employed as apixel value for the virtual ray (Step S18).

In the embodiment, a buffer is generated to obtain the replaced array A2in Step S14. In the following Step S15, a part of data on the array A2is excluded. However, it is also possible that the replaced array A2 isdynamically obtained according to the advance of the virtual ray inresponse to instructions from a user by GUI, and a part of data isexcluded progressively. Thereby, the user can precisely observe anobject while changing the target object and the direction to observe theobject.

FIG. 23 shows a flow chart of the case when whether data is to beexcluded is determined using a change in voxel data in the imageprocessing method of the present embodiment. At first, as shown in FIGS.16A and 16B, the permissible change threshold value TD is determined fordetermining the magnitude of change in voxel data (gradient in graph)(Step S21).

Next, scanning on the array A2 is started (Step S22), and scanning isconducted using a position P on the array A2 (Step S23). Then, a valueV0 at the position (P−1) on the array A2, a value V1 at the position Pon the array A2, and a value V2 at the position (P+1) on the array A2are respectively obtained (Step S24).

Next, change D0=|V1−V0| and change D1=|V2−V1| are respectively obtained(Step S25), and the change D0 and the change D1 are compared with thepermissible change threshold value TD in magnitude (Step S26). Then,when the change D0 and the change D1 are larger than the permissiblechange threshold value TD (yes), the point on the position P is excluded(Step S27).

On the other hand, when the change D0 and the change D1 are not largerthan the permissible change threshold value TD (no), whether theposition P reaches the end of the array or not is determined (Step S28).When the position P is not at the end of the array (no), the position Pis incremented by 1 (Step S30), and the processing of and after the StepS24 are repeated. On the other hand, in Step S28, when the position Preaches the end of the array (yes), the scanning on the array A2 iscompleted (Step S29).

Thereby, the center part of the calcified region is not rendered, andonly the rim, i.e., the outline is rendered. Furthermore, thebloodstream existing in front of and at the back of the center part ofthe calcified region can be rendered. Moreover, the permissible changethreshold value TD can be dynamically changed though GUI by a user, andthe user can change the degree of removal of the calcified region whileviewing the displayed image.

FIG. 24 shows a flow chart of the case when exclusion of data isdetermined using gradient in the image processing method of the presentembodiment. In this case, as shown in FIGS. 4A and 4B, at first, thevirtual ray vector D is obtained (Step S41), and a permissibleparallelism threshold value TP is determined (Step S42).

Next, scanning is started on the array A2 (Step S43), and the scanningis conducted using the position P on the array A2 (Step S44). Moreover,a gradient vector G is obtained based on voxels on the volume datacorresponding to the position P on the array A2 (Step S45).

Next, the absolute value of an inner product of the virtual ray vector Dand the gradient vector G of the volume data is compared with thepermissible parallelism threshold value TP (Step S46). When the absolutevalue of the inner product of the virtual ray vector D and the gradientvector G is larger than the permissible parallelism threshold value TP(yes), the point on the position P is excluded (Step S47).

On the other hand, when the absolute value of the inner product of thevirtual ray vector D and the gradient vector G is not larger than thepermissible parallelism threshold value TP (no), whether the position Preaches the end of the array or not is determined (Step S48). When theposition P is not at the end of array (no), the position P isincremented by +1 (Step S50), and the processes of and after Step 45 arerepeated. On the other hand, when the position P reaches the end of thearray (yes), the scanning on the array A2 is completed (Step S49).

Thereby, the portion where the gradient vector G is substantiallyparallel to the virtual ray vector D of the virtual ray is determined tobe the center part of the calcified region, and thus that portion isexcluded from display data. On the other hand, the portion where thegradient vector G is substantially perpendicular to the virtual rayvector D is determined to be the rim of the calcified region, and thusthat portion is employed as display data. Moreover, the permissibleparallelism threshold value TP can be dynamically changed though GUI bya user. The user can change the degree of removal of the calcifiedregion while viewing the displayed image.

Seventh Embodiment (Ray Casting Method—Gradient is Used)

Incidentally, in ray casting method, opacity can be set for each voxel.Thus, opacity can be assigned to the data on the virtual ray withoutexcluding some range on the virtual ray from display data.

According to the above, for example, it becomes possible to display aportion having a sharp gradient of voxel values as a hard portion, and aportion having a gradual gradient of voxel values as a soft portion, andthe like. Moreover, thereby, it becomes possible to select and notdisplay the portion having a sharp gradient. Furthermore, since itbecomes possible to detect the portion having a sharp gradient (boundarysurface of calcification), an image in which the calcified portion isremoved can be displayed without masking process.

FIG. 25 shows an explanatory drawing of the case when an opacity of theobstructing region is changed and the center part of the obstructingregion is not displayed (Application 1 for ray casting method). Also bychanging the opacity of voxel values of the rendering object, a regionto be an obstruction can be excluded. That is, an exclusion degree a ofthe rendering object is calculated, and the exclusion degree a isassociated with the opacity.

In this case, inner product of a virtual ray vector X−O of theflipped-over data and the gradient vector G is used for calculating theexclusion degree α=|G*(X−O)|. Since a portion where the virtual rayvector X−O and the gradient vector G is parallel and the exclusiondegree α is high is the central part of the calcified region, theopacity of the voxel values are made low so as not to display thecalcified region. Thereby, without performing processes such ascalculation for flip-over, it is possible to display the rim of thecalcified region and the bloodstream in front of and at the back of thecalcified region, and not display the center part of the calcifiedregion.

FIG. 26 shows a flow chart of the case when the opacity is changed usinggradient of voxel values (Application 1 for ray casting method). Thisprocessing is a calculation for each pixel on the screen, and thefollowing calculation is conducted for all the pixels on the image.First, from a projection position, a projection starting point O(x,y,z)and a sampling interval ΔS(x,y,z) are set (Step S101).

Next, a reflecting light E is initialized as “0”, a remaining light I as“1”, and a current calculation position X(x,y,z) as “O” (Step S102).Then, from voxel data in the neighbor of the current calculationposition X(x,y,z), an interpolated voxel value V of the currentcalculation position X is obtained (Step S103). In addition, an opacityα corresponding to the interpolated voxel value V is obtained (StepS104). In this case, by α1=f(V), α2=1−G*(X−O), α=α1*(1−α(2), theexclusion degree at the position X is calculated (Step S112).

Next, a color value C corresponding to the interpolated voxel value V isobtained (Step S105). Then, from voxel data in the neighbor of theposition X(x,y,z), a gradient G of the position X is obtained. From aray direction X−O and the gradient G, a shading coefficient β isdetermined (Step S106).

Next, an attenuated light D (D=I*α) and a partial reflecting light F(F=β*D*C) at the position X(x,y,z) are calculated (Step S107). Then, thereflecting light E and the remaining light I are updated (I=I−D, E=E+F)(Step S108).

Next, whether the current calculation position X reaches a finalposition or not, and whether the remaining light I is “0” or not aredetermined (Step S109). When the current calculation position X is notat the final position and the remaining light I is not “0” (no),ΔS(x,y,z) is added to X(x,y,z), the current calculation position ismoved on (Step S110), and the processes of and after Step S103 arerepeated.

On the other hand, when the current calculation position X reaches thefinal position, or the remaining light I becomes “0” (yes), thecalculation is completed by employing the reflecting light E as a pixelvalue of the pixel under calculation (Step 111). In ray casting methodof the related art, a shading coefficient obtained from a product of agradient of voxel and a light source direction is used for calculating areflecting light amount. However, as described above, in the presentinvention, it is possible to detect a calcified region and atwo-dimensional boundary surface thereof from a product of the gradientand the virtual ray direction, apart from the physical meaning ofgradient. Furthermore, by changing the opacity of voxels according tothe detected information, it is possible to display the rim of thecalcified region and the bloodstream in front of and at the back of thecalcified region without displaying the center part of the calcifiedregion.

Eighth Embodiment

(Ray Casting Method—with Flip-Over)

FIG. 27 shows an explanatory drawing of the case when the opacity ischanged (Application 2 for ray casting method). In this case, theexclusion degree a is obtained from the change in the flipped-over voxelvalues. That is, in the portion to be determined for exclusion, as forflipped-over data, a portion where voxel values are remarkably changedand flipped-over is represented as α=1, a portion where voxel values areremarkably changed but not flipped-over is represented as α=0.5, and theother portion is represented as α=0, for example. Then, the region whereα=1 is not displayed, being determined as an obstructing region. Theregion where α=0.5 is displayed as the rim of the obstructing region.Thus, by obtaining the exclusion degree α from the change in theflipped-over voxel values, it is possible to display the rim of thecalcified region and the bloodstream in front of and at the back of thecalcified region without displaying the center part of the calcifiedregion.

FIG. 28 shows a flow chart of the case when the opacity is changed usingthe change in voxel values (Application 2 for ray casting method). Thisprocessing is a calculation for each pixel on the screen, and thefollowing calculation is conducted for all the pixels on the image.First, from a projection position, a projection starting point O(x,y,z)and a sampling interval ΔS(x,y,z) are set (Step S121).

Next, a reflecting light E is initialized as “0”, a remaining light I as“1”, and a current calculation position X(x,y,z) as “O” (Step S122).Then, from voxel data in the neighbor of the position X(x,y,z), aninterpolated voxel value V of the position X is obtained (Step S123). Inaddition, an opacity α considering the interpolated voxel value V and“exclusion” is obtained (Step S124). In this case, by α1=f(V), α2=g(X),and α=α1*(1−α2), the exclusion degree of the position X is calculated(Step S132).

Next, a color value C corresponding to the interpolated voxel value V isobtained (Step S125). Then, from voxel data in the neighbor of theposition X(x,y,z), a gradient G of the position X is obtained. From aray direction X−O and the gradient G, a shading coefficient β isobtained (Step S126).

Next, an attenuated light D (D=I*α) and a partial reflecting light F(F=β*D*C) at the position X(x,y,z) are calculated (Step S127). Then, thereflecting light E and the remaining light I are updated (I=I−D, E=E+F)(Step S128).

Next, whether X reaches a final position or not, and whether theremaining light I is “0” or not are determined (Step S129). When X isnot at the final position and the remaining light I is not “0” (no),ΔS(x,y,z) is added to X(x,y,z), the current calculation position ismoved on (Step S130), and the processes of and after Step S123 arerepeated.

On the other hand, when X reaches the final position or the remaininglight I becomes “0” (yes), the calculation is completed by employing thereflecting light E as a pixel value of the pixel under calculation (Step131). Thus, by changing the opacity using the change in voxel values, itis possible to display the rim of the calcified region and thebloodstream in front of and at the back of the calcified region withoutdisplaying the center part of the calcified region.

Ninth Embodiment

(MIP Processing—with Gradient and Flip-Over)

FIG. 29 shows a flow chart of MIP processing in which gradientprocessing and flip-over processing are conducted. This processing is acalculation for each pixel on the screen, and the following calculationis conducted for all the pixels on the image. First, from a projectionposition, a projection starting point O(x,y,z) and a sampling intervalΔS(x,y,z) are set (Step S141).

Next, a maximum value M is initialized as a minimum value of the system,and a current calculation position X(x,y,z) as “O” (Step S142). Aninterpolated voxel value V of the position X(x,y,z) is obtained fromvoxel data in neighbor of the position X (Step S143). Then, voxel dataVS1 in neighbor of the position X(x,y,z) is obtained (Step S144), and aflipped-over data VS2 is calculated from VS1 (Step S145).

Next, gradient G at the position X of the flipped-over data VS2 iscalculated (Step S146), and an inner product I of ray direction X−O andthe gradient G is obtained (Step S147). Then, whether the condition of−T<I<T is satisfied or not for a threshold value T is determined (StepS148). When the condition is satisfied (yes), the maximum value M iscompared with the interpolated voxel value V (Step S149). When themaximum value M is smaller than the interpolated voxel value V (yes),the interpolated voxel value V is assigned to the maximum value M as anew Maximum value (Step S150).

Next, whether X reaches a final position or not is determined (StepS151). When X is not at the final position (no), ΔS(x,y,z) is added toX(x,y,z), the current calculation position is moved on (Step S152), andthe processes of and after Step S143 are repeated. Moreover, when Xreaches the final position (yes), the maximum value M is employed as apixel value of the pixel under calculation, and the processing iscompleted (Step S153). Thus, by combining the gradient processing andthe flip-over processing, it is possible to display the rim of thecalcified region and the bloodstream in front of and at the back of thecalcified region without displaying the center part of the calcifiedregion.

The image processing method of the embodiments can be conducted by GPU(Graphic Processing Unit). GPU is a processor which is designed to bespecialized particularly in image processing as compared withgeneral-purpose CPU. Usually, GPU is mounted on a computer separatelyfrom CPU.

Moreover, in the image processing method of the embodiments, calculationof volume rendering can be divided by a certain image region, a regionof volume, or the like, and subsequently the divided regions after thecalculation can be superimposed on each other. Accordingly, the imageprocessing method of the present embodiment can be performed by parallelprocessing, network distributed processing, or a combination thereof.

The embodiments of the invention are described in the above, however,the invention is not limited to the above embodiments. The followingexamples are also applied to the invention.

Example 1

For example, a lump having a high contrast in the neighbor of anobservation object is referred to as A, and a vector determined by aprojection method and a projection direction is referred to as B.Herein, B is a viewing direction in the case of parallel projection, aradial vector in the case of VE (virtual endoscope), or a vectororthogonal to the viewing direction.

In this case, in the rendering processing by ray casting, for thepurpose of preventing A from being an obstruction when observing theobserving object, the processing has a mechanism that the outline of Ais particularly emphasized selectively and dynamically in each step ofray casting. Then, according to the emphasized result and B, any of thefollowing processes is performed: (1) skip the step; (2) replaceoriginal data with other values, and process; or (3) change the amountof attenuation in the case of a rendering method wherein attenuation oflight is simulated.

Example 2

A lump having a high contrast in the neighbor of an observation objectis referred to as A, the observation object as B, and a vectordetermined by a projection method and a projection direction as C.Herein, C is a viewing direction in the case of parallel projection, aradial vector in the case of VE, or a vector orthogonal to the viewingdirection.

In this case, in the rendering processing by ray casting, in a situationthat A obstructs the observation of B, the processing has a mechanismthat the outline of A is particularly emphasized selectively anddynamically in each step of ray casting, for the following purposes: (1)the image of B is not deteriorated in the region where B is notoverlapped with A; (2) the region where B overlaps with A is removedfrom or made translucent in the image, except for a part of the image ofA showing the characteristic of A briefly; and (3) the whole view of Bcan be observed without being obstructing by A. Then, according to theemphasized result and C, any of the following processes is performed:(1) skip the step; (2) replace original data with other values, andprocess; or (3) change the amount of attenuation in the case of arendering method wherein attenuation of light is simulated.

Example 3

For example, an application to an air image (volume rendering imagewhich is translucent and where only the outline is easily viewable) of acolon is possible. In the case of the air image, “A=B” and “the regionshowing the characteristic of A briefly=(equals to) the region desiredto be observed of B”.

In this case, a lump having a high contrast in the neighbor of anobservation object is referred to as A, the observation object as B(there is a case that B is identical with A), and a vector determined bya projection method and a projection direction as C. Herein, C is aviewing direction in the case of parallel projection, a radial vector inthe case of VE, or a vector orthogonal to the viewing direction.

In this case, in the rendering processing by ray casting, in a situationthat A obstructs the observation of B, the processing has a mechanismthat the outline of A is particularly emphasized selectively anddynamically in each step of ray casting, for the following purposes: (1)in the region where B is not overlapped with A, the image of B is notdeteriorated to hinder the observation of B; (2) the region where Boverlaps with A is removed from or made translucent in the image, exceptfor a part of the image of A showing the characteristic of A briefly;and (3) the whole view of B can be observed without being obstructed byA. Then, according to the emphasized result and C, any of the followingprocesses is performed: (1) skip the step; (2) replace original datawith other values, and process; or (3) change the amount of attenuationin the case of a rendering method wherein attenuation of light issimulated.

Example 4

A lump having a high contrast in the neighbor of an observation objectis referred to as A, the observation object as B (there is a case that Bis identical with A), and a vector determined by a projection method anda projection direction as C. Herein, C is a viewing direction in thecase of parallel projection, a radial vector in the case of VE, or avector orthogonal to the viewing direction.

In this case, in the rendering processing by ray casting, in a situationthat A obstructs the observation of B, the processing has a mechanismthat the outline of A is particularly emphasized selectively anddynamically in each step of ray casting, for the following purposes: (1)in the region where B is not overlapped with A, the image of B is notdeteriorated to hinder the observation of B; (2) the region where Boverlaps with A is removed from or made translucent in the image, exceptfor a part of the image of A showing the characteristic of A briefly;and (3) the whole view of B can be observed without being obstructed byA. Then, when the emphasized result and C satisfy a certain conditionwhich is set beforehand, any of the following processes is performed asan alternative to usual processing: (1) skip the step; (2) replaceoriginal data with other values, and perform usual processing; (3)perform processing to which a transformation with the emphasized resultand C is added as a preprocessing of usual processing; or (4) change theamount of attenuation in the case of a rendering method whereinattenuation of light is simulated.

Example 5

In the above examples, it is assumed that the outline of A is left inthe result image. However, an embodiment in which the outline of A isnot left is also possible. In this case, a lump having a high contrastin the neighbor of an observation object is referred to as A, theobservation object as B (there is a case that B is identical with A),and a vector determined by a projection method and a projectiondirection as C. Herein, C is a viewing direction in the case of parallelprojection, a radial vector in the case of VE, or a vector orthogonal tothe viewing direction.

In this case, in the rendering processing by ray casting, in a situationthat A obstructs the observation of B, the processing has a mechanismthat the outline of A is particularly emphasized selectively anddynamically in each step of ray casting, for the following purposes: (1)in the region where B is not overlapped with A, the image of B is notdeteriorated to hinder the observation of B; (2) in the region where Boverlaps with A, (2.1) regions other than a part of the projection imageof A showing the characteristic of A briefly or (2.2) all of theprojection image of A, is removed from or made translucent in the image;(3) the whole view of B can be observed without being obstructed by A.Then, when “the original data, the emphasized result and C” at theposition in process of each step of ray casting and in the neighbor theposition satisfy a certain condition which is set beforehand, any of thefollowing processes is performed as an alternative to usual processing:(1) skip the step; (2) replace original data with other values, andperform usual processing; (3) perform processing to which atransformation with the emphasized result and C is added as apreprocessing of usual processing; or (4) change the amount ofattenuation in the case of a rendering method wherein attenuation oflight is simulated.

Example 6

The image processing method of the invention comprises (A) in volumerendering, (B) determining whether the value of a sample point of anobservation object is employed or not (excluded or not) in each samplepoint on the virtual ray. In this example, (C) an object of theinvention is to render only the outline of an obstructing region.Moreover, (D) it is desirable to use data generated from volume data fordetermination of exclusion (this is not indispensable).

In related arts, an obstructing region is removed by modifying a volumeor generating a mask volume. In the invention, an obstructing region isdynamically calculated at the time when the virtual ray is projected.Accordingly, an image in which the projection direction is consideredcan be generated (extraction of the outline from the direction of theviewpoint). Therefore, since the display of the outline changes as thedirection of the viewpoint is changed, a desirable image is easilyobtained.

(A) The image processing method of the invention can be used in generalvolume rendering, and is effectively utilized particularly in MIP methodand ray casting method. In volume rendering, there are methods such as(A1) MIP method and MinIP method, represented by MIP method, wherein onesample point is selected from the data on the virtual ray to determine apixel. Furthermore, there are methods such as (A2) ray casting method,Raysum method and average value method, represented by ray castingmethod, wherein a plurality of sample points on the virtual ray areselected to determine a pixel.

The present invention can be applied to any of the methods. In (A1), thepresent method can be implemented by, after (B), excluding the samplepoints selected in (B) from the data on the virtual ray (animplementation example in MIP method). On the other hand, in (A2), thepresent method can be implemented by, after (B), excluding the samplepoints selected in (B) from the plurality of points on the virtual ray.

Moreover, in the method of (A2) such as ray casting method, though (A),(B) and (C) can be implemented as they are as described above, thesample points are not necessarily excluded, and the degree ofcontribution on the determination of pixels can be lowered.

For example, in ray casting method, an opacity α is calculated based onvoxel values for sample points through which the virtual ray passes. Thevalue of opacity α can be manipulated for the sample points selected in(B).

(B) In the image processing method of the invention, it is determinedwhether the value of a sample point of an observation object is employedor not (excluded or not) in each sample point on the virtual ray. Forthe determination of exclusion, in addition to the above method (B1)using gradient (note: for flipped-over data, gradient may be calculatedfor all the sample points or for a part thereof (such as in the neighborof the threshold value)) and the method (B2) using a change such as adifference of voxel values, following methods can be used: (a) excludinghigh-frequency components through frequency analysis; (b) calculatingvariance of data and excluding sample points having the variance equalto or larger than a threshold value; (c) using a noise removal filter;and the like. Moreover, the exclusion may be determined by not onlybinary values of “exclude” and “not exclude” but also multiple valueswith an “exclusion degree”.

(C) In the image processing method of the invention, an object of theinvention is to render only the outline of an obstructing region. Inaddition, two-dimensional characteristics of data can be displayed, notbeing limited to the outline. Moreover, by dynamically performingcalculation for the exclusion irrespective of the outline, it ispossible to generate a new image in medical imaging.

(D) Moreover, in the image processing method of the invention, it isdesirable to use data generated from volume data for determination ofexclusion (this is not indispensable). Here, following cases areconsidered: (D1) flipped-over data is used; (D2) data below a thresholdvale is used (when gradient is used); (D3) When gradient is used, thesame effect can be obtained without flip-over by determining (1) toexclude when a voxel value is considerably larger than a thresholdvalue, (2) to exclude according to gradient when a voxel value is closeto the threshold value, and (3) not to exclude when a voxel value isconsiderably smaller than the threshold value;

(D4) mask data is used, that is, the same effect can be obtained bycalculating gradient on the mask data; and (D5) data generated from thevolume data may be (a) calculated at the time when a virtual ray isprojected, or (b) generated as second volume data beforehand.

In the above embodiments, determination of a calcified region isconducted. However, this invention may be applied to the determinationof any region as long as it is an obstructing region. For example, it isparticularly effective for observation of medical devices such asstents, medical clips, and medical coils, which are inserted in a humanbody. Moreover, the region may be a region of bloodstream or an organ.Furthermore, the region may be a contrast region where signals areemphasized by a contrast agent. The contrast region includes a contrastregion by a low concentration contrasting. In addition, the obstructingregion in not limited to a high-signal region, and may be a low-signalregion such as an air bubble so that a two-dimensional outline thereofcan be displayed. Moreover, the obstructing region may be a regionhaving medium signal intensity such as fat so that a two-dimensionaloutline thereof can be displayed.

In the above embodiments, the obstructing region is a lump region.However, the region may be of any form as long as it is an obstructingregion. For example, in an image in which bloodstreams having differentcircumferences are intricately crossed with each other as shown in FIG.30A, the bloodstreams can be accurately recognized by displaying thetwo-dimensional outline of the bloodstreams as shown in FIG. 30B.Moreover, in the case of radiographic contrast of intestines, thepositional relationship and the shape of the intestinal wall foldedintricately can be well recognized by displaying the two-dimensionaloutline of the intestines.

In the above embodiments, the image according to the invention is solelydisplayed but the image according to the invention may be displayed incombination with the other images. For example, the image according tothe invention and the image generated by the related art may bedisplayed together being side by side or overlapped. In this case, theimages by the same angle of view, parallel projection method,perspective projection method, or cylindrical projection method, anddata on the virtual ray may be obtained at once or sequentiallyaccording to the necessity of calculation.

In the above embodiments, the whole image is generated by the samecalculation method. However, the image processing according to thepresent method may be applied to only a part of the image. For example,the image processing according to the present method may be applied toonly the neighbor of a pointer by a pointing device such as a mouse.Thereby, it becomes easy to observe the image while comparing the imageprocessing according to the related art and the image processingaccording to the present method.

In the above embodiments, the whole image is generated by the samecalculation method. However, the image processing according to thepresent method may be applied to only a part of the image. Particularlyin the present method, for the portion where an obstructing region doesnot exist, the same image can be obtained as that obtained in the methodof the related art. Thus, the processing speed can be increased bygenerating an image by the method of the related art beforehand, anddetermining particularly the region which includes an obstructing regionautomatically. In particular, in the example using MIP method, thepresent method can be applied only to the portion where a maximum valueon the virtual ray is equal to or larger than a threshold value.

In the above embodiments, gradient information is obtained by acquiringthe difference of neighboring 3×3×3 voxel region of a target voxel.However, the method for obtaining the gradient information is notlimited to the above example. For example, in the case when the value ofa point through which the virtual ray passes is obtained by not thetarget voxel but interpolation of neighboring voxels, the neighboring3×3×3 voxel region of the target voxel may be also obtained byinterpolation, respectively. Moreover, not the 3×3×3 voxel region but a2×2×2 voxel region may be used. Furthermore, the gradient may beobtained after normalizing the voxel in the direction of the virtualray.

In the above embodiments, since it is difficult to intuitivelyunderstand the degree of contribution of a voxel on the virtual ray toan image, it is possible to display an image in which the degree ofcontribution of the voxel contained in volume data is separatelyvisualized. For example, it is possible to display a graph which showsthe degree of contribution of a voxel on the virtual ray. Moreover, agraph can be displayed which shows the degree of contribution for aplurality of virtual rays. For example, a CPR (curved multi planarreconstruction) image is generated using planes composed of a group ofvirtual rays corresponding to a line on the image, and the degree ofcontribution of a voxel on a virtual ray relating to each point on theCPR image can be mapped while superimposed.

In the above embodiments, since it is difficult to intuitivelyunderstand whether a voxel on the virtual ray is used or excluded (notused) in the calculation of an image, it is possible to display an imagein which whether the voxel contained in the volume data is used orexcluded in the generating the image is separately visualized. Forexample, it is possible to display a graph which shows whether a voxelon the virtual ray is used or excluded in generating an image. Moreover,a graph can be displayed which shows whether a voxel on the virtual rayis used or excluded in generating an image for a plurality of virtualrays. For example, a CPR (curved multi planar reconstruction) image isgenerated using planes composed of a group of virtual rays correspondingto a line on the image, and whether a voxel on a virtual ray relating toeach point on the CPR image is used or excluded in generating an imagecan be mapped while superimposed.

In the above embodiments, since it is difficult to intuitivelyunderstand how a profile pattern on the virtual ray is replaced, thereplaced profile pattern on the virtual ray may be displayed. Forexample, it is possible to display the replaced profile patterncorresponding to a point pointed by a pointing device, in a same windowby being superimposed on the image generated by the present method, orin another window. Moreover, original profile pattern can be alsodisplayed being superimposed to the replaced profile pattern.

In the above embodiments, particularly in the image processing methodusing only a single point on a virtual ray, since it is difficult tointuitively understand the three-dimensional position where the singlepoint is obtained, the three-dimensional position where the single pointis obtained can be visualized. For example, an image is generated inwhich the three-dimensional position where the single point is obtainedis mapped as depth information. Moreover, the mapped image can bedisplayed by being superimposed on or arranged side by side with animage generated by the present method or a calculation method of therelated art.

In the above embodiments, calculation is conducted by a singleprocessing unit. However, parallel processing may be conducted by aplurality of processing units. For example, in the present method, sincecalculation can be conducted independently for each virtual ray, thecalculation can be conducted for each virtual ray in parallel.

In the above embodiments, the image processing is conducted by a singlecentral processing unit. However, GPU (graphics processing unit)equipped with a programmable shader may be used for the imageprocessing. Moreover, the other processing units may be employed.

In the above embodiments, parameters used in the image processing arepredetermined beforehand. However, the parameters may be dynamicallychanged according to the operation of a user. For example, a thresholdvalue for flip-over processing may be changed by operating a slider seton the screen.

In the above embodiments, a direction vector of the virtual ray is used.However, other direction vectors may be employed. For example, by usinga direction vector which crosses with the direction of the virtual rayobliquely, an effect corresponding to shades can be expressed in theimage generated through MIP processing and the like. Moreover, forexample, by using the direction vector of a central line of a bloodvessel, the region in front of the blood vessel and the region at theback of the blood vessel are displayed differently.

In the above embodiments, MIP method is used as a volume renderingmethod wherein values of at least one point of data on the virtual rayare used in determining the pixel values, and positional relationship ofthe at least one point is mutually exchangeable. However, other methodsmay be employed for the volume rendering method. For example, a MinIP(minimum intensity projection) method wherein a minimum value is usedmay be employed. Moreover, for example, an average value method whereinan average value of at least one point is used may be also employed.Furthermore, for example, a Raysum method wherein a sum of at least onepoint is used may be employed. Furthermore, for example, a Top10MIPmethod may be employed wherein values of top ten points on the virtualray are obtained and an average value thereof is used.

In the above embodiments, gradient at each position in a volume is used.It may be possible to use gradient at a position which is moved from apassing point of the virtual ray in the volume. Thereby, the position onwhich a two-dimensional outline is rendered deviates, and hence moreprecise observation is enabled on the boundary region while suggestingthe presence of the two-dimensional outline to a user.

In the above embodiments, gradient at each position in a volume is used.It may be possible to use gradient information at a position in a volumeother than the volume which is used in determining the pixels. Forexample, by using other volume data which is different in time series,movement of the two-dimensional outline can be rendered. Moreover, forexample, when a mask volume generated beforehand is used, outlineinformation generated by an outline extraction algorithm or a user canbe rendered.

In the above embodiments, a still image is generated, but a moving imagemay also be generated. Moreover, a moving image displayed according tothe operation by a user may be dynamically generated. For example, whena moving image wherein a viewpoint rotates around an observation objectis generated, the two-dimensional outline of the observation object canbe more precisely observed. Furthermore, for example, in the case when amoving image is used wherein parameters relating to the image processingare changed, the two-dimensional outline can be also more preciselyobserved.

In the above embodiments, a two-dimensional outline of an observationobject is displayed, but the display of the observation object is notlimited to the two-dimensional outline. For example, when a portionwhere the angle between the direction vector of the virtual ray and thegradient vector is small is not excluded but a portion where the angleis large is excluded, the center part of the observation object isemphasized. Moreover, for example, when the determination is performedby the direction of the outer product vector of the direction vector ofthe virtual ray and the gradient vector, those directing to a specificdirection can be displayed among the two-dimensional outlines.

In the above embodiments, opacity is set when ray casting method isused, but any degree of contribution on an image can be set withoutlimiting to opacity. For example, in the average value method, an imagecan be generated by setting the degree of contribution and using aweighted average instead of the average value.

In the above embodiments, a two-dimensional outline of a calcifiedregion is displayed, but objects to be displayed are not limited to acalcified region. For example, it is particularly effective forobservation of medical devices such as stents, medical clips and medicalcoils, which are inserted in a human body. Moreover, without limiting toa high-signal region, a two-dimensional outline of a low-signal regionsuch as an air bubble may be displayed.

In the above embodiments, volume data obtained from a CT apparatus isused, but the volume data may be obtained by any method or apparatus.For example, volume data obtained from an MRI (magnetic resonanceimaging) apparatus or a PET (positron emission tomography) apparatus maybe used. Moreover, volume data modified by applying a filtering processor the like to the volume data, or volume data obtained by combining aplurality of volume data may be also employed.

In the above embodiments, the angle between the first vector informationand the second vector information is calculated, but the angle may be anegative value or may be larger than 90 degrees. Thereby, only a wall ata front side can be displayed.

The present invention is applicable to an image processing methodcapable of rendering an image by removing an obstructing region such asa calcified region and determining the outline of the obstructing regiondynamically, during calculation of a medical image such as MIPprocessing.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the described preferredembodiments of the present invention without departing from the spiritor scope of the invention. Thus, it is intended that the presentinvention cover all modifications and variations of this inventionconsistent with the scope of the appended claims and their equivalents.

1. An image processing method by volume rendering, said image processingmethod comprising: selecting at least one point which is aligned on avirtual ray; determining a degree of contribution of each value of theselected at least one point; and determining a pixel value of an imageby the volume rendering based on the determined degree of contributionand said each value of the selected at least one point.
 2. The imageprocessing method according to claim 1, wherein at least one of thedegree of contribution is zero.
 3. The image processing method accordingto claim 1, wherein the degree of contribution is determined based ondata obtained by replacing original data on the virtual ray.
 4. Theimage processing method according to claim 3, wherein values of thereplaced data are obtained by flipping values of the original data overat a threshold value.
 5. The image processing method according to claim1, wherein the degree of contribution is determined further based on agradient vector which is on a volume and corresponds to a position ofthe selected point, and a direction vector of the virtual ray.
 6. Theimage processing method according to claim 1, wherein the degree ofcontribution is determined further based on a change of voxel values onthe virtual ray.
 7. The image processing method according to claim 1,further comprising: displaying a two-dimensional outline of a regionincluded in a rendering object on the volume rendering image.
 8. Theimage processing method according to claim 1, further comprising:displaying excluding a region included in a rendering object on thevolume rendering image.
 9. The image processing method according toclaim 1, wherein the volume rendering image and an another image aredisplayed arranged in side by side, being overlapped with each other, orby showing a difference of the images.
 10. The image processing methodaccording to claim 1, wherein the pixel value is determined only for aregion which is designated by a user.
 11. The image processing methodaccording to claim 1, wherein the pixel value is determined only for awindow provided on a screen.
 12. The image processing method accordingto claim 7, wherein the outline is displayed while continuously changed.13. The image processing method according to claims 1, wherein the imageprocessing is performed by parallel processing.
 14. The image processingmethod according to claim 1, wherein the image processing is performedby a GPU (graphics processing unit).
 15. The image processing methodaccording to claim 1, wherein the image processing is performed by a GUI(graphical user interface) in which parameters are changeable.
 16. Theimage processing method according to claim 1, wherein the imageprocessing is performed by MIP (Maximum Intensity Projection) method,MinIP (Minimum Intensity Projection) method, Raysum method, an averagevalue method or ray casting method.